High-frequency generator for performing high-frequency surgery having adjustable power limitation, and method for controlling the power limitation

ABSTRACT

The invention relates to an HF generator and to a method for limiting the output effective power of the HF generator, especially for performing HF surgical cutting and coagulation of human or animal tissue. According to the invention, the output voltage and the HF output current of the HF generator are detected by at least two detector devices, the peak values and the effective values of the HF output voltage and of the output current as well as the mean value of the output effective power of the HF generator are determined by an evaluation device, and the calculated mean value is compared to a defined maximum mean value of the output effective power of the HF generator by a comparison device. Afterwards, a modulation device modulates the HF output voltage using a pulse-shaped modulation signal. A control device controls the modulation device in such a manner that the pulse duration of the pulse-shaped modulation signal and/or the pulse duration between the pulse-shaped modulation signals is modified in order to keep the peak value of the output voltage constant when the calculated mean value of the output effective power is greater than the maximum mean value of the output effective power. Alternatively, the HF generator is equipped with a sensor for evaluating the intensity of electric arcs between an electrode connected to the HF generator and the tissue.

BACKGROUND

The present invention relates to a high-frequency (HF) generator with anadjustable means of limiting the effective output power, intended inparticular for HF surgical procedures including the cutting andcoagulation of human or animal tissue, and also relates to a method oflimiting the output power of the high-frequency generator.

In high-frequency surgery, the HF generators used for cutting andcoagulating human or animal tissue are distinguished by the fact thatthey provide a HF voltage which, when applied to an electrode, producesan electrical arc between electrode and tissue that has the effect ofmaking an incision in the tissue. The HF voltages between electrode andtissue that are required for this purpose have a minimal value of ca.200 Vp (volt peak). During such cutting and coagulation, the voltageapplied between the electrode and the tissue has a decisive influence onthe amount of coagulation at the edges of the incision. To keep thecoagulation degree constant, HF generators are provided with a controlcircuit that regulates the HF output voltage from the HF generator, andhence the intensity of the electrical arc between electrode and tissue,so as to maintain a constant voltage or intensity level.

The effective output power, which depends on the output voltage of theHF generator, is given by the equation

$P = \frac{U^{2}}{R}$and hence is also a function of the load impedance R. If the loadimpedance is reduced, owing for instance to a large incision area, theresult is that the output voltage U can be kept constant only for aslong as the HF generator can generate the necessary effective outputpower. As soon as the HF generator reaches its power limit, it is nolonger capable of maintaining the desired constancy of the outputvoltage U or the intensity of the electrical arc. This is the case inparticular when the effective output power (or output current) of the HFgenerator reaches a maximum, i.e. an upper limit that has been specifiedby a prior adjustment process.

For the cutting and coagulation effect, the peak value of the outputvoltage is particularly crucial. As is well known, the peak value of theHF voltage must reach at least 200 V, so as to ignite the electrical arcrequired in order to achieve the cutting effect.

In known HF generators the HF output voltage is automatically reducedwhen the prespecified maximal output power is reached or exceeded, sothat the effective output power of the generator does not increasebeyond the generator's power limit. At the same time, however, an effortshould be made to keep constant the peak value of the output voltage,and hence the intensity of the electrical arc, so that the outputvoltage will have a largely constant influence on the degree of scabproduction at the edges of the incision.

SUMMARY

The teachings of the present invention make available a HF generator forthe surgical cutting and coagulation of tissue, as well as a method oflimiting the effective output power thereof, that make it possible tokeep the degree of scab production at the edges of the incisionsubstantially constant even in the case of large-area, deep and/orextremely rapid cutting.

Preferably in the HF generator, and in the method for limiting theeffective output power thereof, the generator's output voltage and HFoutput current are detected and these values are used to calculate themean effective output power, which is then compared with the previouslyspecified, adjustable maximal mean value of the effective output power,and if the calculated mean is found to be greater than the presetmaximal mean value, the HF output voltage is modulated by means of apulsed modulation signal. The duration of the modulation pulses and/orthe duration of the intervals between them are/is preferably adjustedsuch that the peak value of the HF output voltage, and hence theintensity of the electrical arc, is kept constant. Because themodulation signal subdivides the voltage supply into pulses, the meaneffective value of the HF output voltage is reduced as the duration ofthe intervals separating the individual pulses is increased; as aresult, the effective output power of the HF generator is also reduced,and thus the HF generator is advantageously kept within its powerlimits.

For further details regarding the procedure for keeping the intensity ofthe arc constant, reference is made to the applicant's unexamined Germanapplications DE 198 39 826 A1 and DE 38 05 291 A1.

At the same time, the peak value of the HF output voltage, and hence theintensity of the arc within the duration of the pulse, is kept at aconstant level, as a result of which the degree of coagulation at theincision edges can be made to remain the same even if the load impedancebecomes reduced.

Alternatively, the mean value of the effective output power can also becalculated on the basis of the power delivered from the mains supply anda known value for the efficiency of the HF generator.

Instead of a reduction in the degree of coagulation such as occurs whenthe load impedance becomes less owing to very deep or rapid cutting orthe like, the surgeon will detect a mechanical resistance to progress ofthe electrode, inasmuch as a pulsed signal appears or the existingpulses become separated by longer intervals. As a result, although thecutting movement is slowed down, the desired degree of coagulation atthe edges of the incision is still maintained.

This mechanical resistance, in combination with the preservation of thedesired degree of coagulation, makes it impossible to carry out thecutting movement too rapidly, which ensures sufficiently goodcoagulation at the incision edges and consequently a cessation ofbleeding.

So that the evaluation device can accurately calculate the meaneffective output power, the phase shift between the output voltage andthe output current is also measured and taken into account.

Preferably the duration of the pulses or the intervals between them isallowed to change by no more than a specified amount, because a deviceis provided that imposes a minimal and a maximal permissible pulseduration or interval duration. This measure ensures, firstly, that no HFoutput voltage is produced with a pulse duration so short that thecutting effect is thereby impaired. Secondly, the imposition of amaximal pulse duration ensures that the pulsed output voltage producedby the modulation signal never exceeds a continuously oscillating HFoutput voltage.

So as to obtain an effectively regulated HF generator and a method oflimiting the effective output power of the HF generator that can bepractically implemented, the peak values of the HF output voltage, orthe intensity of the electrical arc, and the maximal permissibleduration of the pulses and/or intervals are specified at the outset byan initialization device, before the control circuit is put intooperation.

A mains power supply, which provides the HF generator with more power assoon as the calculated mean effective output power becomes the same asor greater than the specified and preset maximal mean value, ispreferably connected to the control device. This arrangement allowscompensation for the generator's internal resistance whilesimultaneously keeping the peak value of the HF output voltage at thesame level.

It is advantageous for the durations of pulses or intervals betweenpulses to be within a range from 3 μs (at 330 kHz) to 200 ms, so as toensure that the effects described above regarding the minimal andmaximal permissible pulse and/or interval durations are achieved.

It is also possible to lower the set point in the HF generator thatgoverns the peak HF output voltage, and hence the intensity of the arc,whenever the pulse duration falls below the minimal permissibleduration. As a result, at the next cycle through a control circuit inthe HF generator it is possible effectively to regulate the limitationon the effective output power of the HF generator, in such a way thatthe process of cutting through the tissue can be continued.

However, the set point for the peak value of the HF output voltage, orthe intensity of the electrical arc, can also be raised, namely when thecalculated mean value is below the specified maximal mean effectiveoutput power, and when the set point has a value smaller than apre-adjusted peak value. By this means the control circuit of theHF-generator can be readjusted, as far as the upper power limit of thegenerator.

If the set point is above the prespecified peak value, the pulseduration is increased, as long as it remains below the maximalpermissible pulse duration.

In case the maximal effective output power of the generator is limited,by a prior adjustment, to a level below the generator's actual powerlimit, the effective output power of the HF generator is indicated as amean effective power value, obtained by averaging over an integrationtime. This integration time can be equal to the modulation period, i.e.the duration of one modulation cycle, or can be an integral multiplethereof.

It is also advantageous for the effective output power to be calculatedon the basis of the output power of the mains power supply and theefficiency of the HF generator.

Additional details, advantages and further developments of the inventionwill be evident from the following description of an exemplaryembodiment with reference to the drawings, wherein

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary embodiment of the HFgenerator,

FIG. 2 is a plot to illustrate the relationship between the effectiveoutput power of the HF generator and the load impedance R,

FIGS. 3A–3C comprise diagrams in which the HF output voltage of the HFgenerator is plotted as a function of time for various pulse durations,and

FIG. 4 is a flow diagram of the control circuit of the HF generator andof an exemplary embodiment of the method.

DETAILED DESCRIPTION OF THE PREFERRED EXAMPLES

FIG. 1 shows schematically an exemplary embodiment of the HF generatorthat comprises, in addition to the HF generator 1 itself, two detectordevices 2, 3 with which to measure the output voltage or output currentof the HF generator 1. For this purpose, the detector devices 2, 3 areconstructed as sensors, namely a voltage sensor disposed in parallelwith the HF generator 1 and a current-intensity sensor 3 in series withthe HF generator 1 and with a load impedance 9.

The sensors 2 and 3 are both connected to an evaluation device 4, whichevaluates the sensor signals in order to find the peak and/or effectivevalues of the HF output voltage and the peak and/or effective values ofthe output current. After these values have been obtained, theevaluation device calculates the mean effective output power of the HFgenerator, by multiplying the effective value of the HF voltage by thatof the HF current and by a cosine function of an additionally measuredphase angle between the HF output voltage and the HF output current.

Subsequently the mean value so calculated is sent to a comparison device5, where it is compared with a previously specified and preset maximalmean value for the effective output power of the HF generator, todetermine whether the calculated mean value is larger or smaller thanthe maximal mean value.

The modulation device 7 then produces a pulsed modulation signal tomodulate the output voltage of the HF generator 1 whenever thecalculated output power exceeds the preset reference value for theoutput power.

If the calculated mean value is greater than the maximal mean value, acontrol device 6 is activated, which controls the modulation device 7for modulating the output voltage with a pulsed modulation signal. Thecontrol is such that the duration of the pulses in the modulation signalis reduced in order to obtain, so to speak, a “dilution” of the outputvoltage. In this process the pulse duration is lowered by whateveramount is necessary in order to keep constant the peak value of theoutput voltage.

The control device 6 simultaneously controls a mains power source 8 thatsupplies the HF generator 1. Hence it is additionally possible to alterthe input power signals to the HF generator so as, for example, to bringabout compensation for the generator's internal resistance.

In FIG. 2 the time course of the effective output power 15 is shown as afunction of the load impedance. In this diagram it is evident that abovea critical load impedance R_(crit) the HF generator does not reach itspower limit, so that the peak value of the output voltage of the HFgenerator can be kept constant. This applies likewise to the state inwhich the load impedance R coincides exactly with the value of thecritical impedance R_(crit).

However, as soon as the load impedance R becomes smaller than thecritical load impedance R_(crit), the HF generator comes up to the limitof the power it can produce, i.e. its maximal power 16. Therefore, inaccordance with the invention, the effective output power of the HFgenerator is kept constant at a maximal permissible value 16 in that theratio of peak value to effective value of the output voltage of the HFgenerator (the “crest factor”) is appreciably changed as the loadimpedance becomes smaller.

In FIG. 3 this variability in the ratio of peak to effective values ofthe output voltage signals is explained in greater detail by threediagrams. The three diagrams A, B, C differ from one another inasmuch asdiagram A shows the time course of a continuously oscillating HF outputvoltage, whereas in diagrams B and C the output voltage oscillation issubdivided into pulses, the durations of which are different in the twodiagrams.

The continuously oscillating HF output voltage 13 shown in diagram A,for a load impedance above R_(crit), has a peak value 11 and aneffective value 12. When this continuously oscillating signal ismodulated, in the case of load impedances below R_(crit), the effectivevalue 12 of the HF output voltage is reduced while the peak value 11 ofthe HF output voltage remains the same as previously, as shown forinstance in diagram B. The pulses 14 in the modulation signal in diagramB have a duration 14 a, which would be the case when the load impedanceis only slightly below R_(crit).

In diagram C the modulation signal has a pulse duration (14 a)applicable to a load impedance R that has been still further reduced.The effective value 12 of the HF output voltage has become even lowerthan before, whereas the peak value 11 still remains constant within anoscillation pulse having the specified duration. With such a combinationof lowered effective value and constant peak value, on the basis of thecrest factor it is possible for the effective output power to be keptbelow a specified mean value.

The diagrams A, B and C show the time course of the HF output voltage 10over a time period of 200 ms, from which it can be discerned that thepulse durations are preferably within a range from 3 μs (at 330 kHz) to200 ms.

FIG. 4 shows a flow diagram representing the sequence of events in thecontrol circuit associated with the HF generator and with the method.

Before the actual control circuit (loop) is entered, by means of aninitialization device the parameter UPpsp, which prespecifies thedesired peak value of the HF output voltage to achieve the desiredregulation of the generator, is used to initialize a set-point valueUPspt to be used during this regulation.

The maximal permissible pulse duration, PDmax, likewise serves toinitialize the actual pulse duration, PDact. If the user does not wantthe output voltage to be modulated by dividing it into pulses, butrather is aiming for a continuous, unmodulated output voltage, thenPDmax can be made equal to the period of the voltage oscillation.

Once the actual control circuit has come into operation, it can beinactivated, for example by releasing a finger switch. Insofar as suchinactivation does not occur, by means of the detector devices 2, 3 andthe signal-evaluation device 4 the peak value Up and the effective valueUeff of the output voltage are measured, as also are the effective valueIeff of the output current and the phase angle PHI between the outputvoltage and the output current intensity.

Then the peak value Up of the output voltage is regulated to the setpoint UPspt of the HF output voltage. In a subsequent step the meanvalue Pavg of the effective output power of the HF generator over atleast one cycle of the modulated signal is calculated by multiplyingtogether the effective values of the HF output voltage and the outputcurrent, with the cosine of the phase angle PHI, and in a further stepthis calculated mean value Pavg is compared with a previously specifiedmaximal mean value Pmax.

Whenever the maximal mean value Pmax is exceeded by the calculated meanvalue Pavg, the result is an incremental reduction of the momentarypulse duration PDact, or if the output is not currently being modulated,then a pulsed modulation signal with the appropriate pulse durationbegins to be applied. In the process, the duration of the shortenedpulses must not be allowed to fall below a minimal permissible valuePDmin. If a reduction would cause the pulse duration to become less thanPDmin, then instead of reducing the pulse duration a reduction of thepeak-voltage set point UPspt is carried out, after which there isanother cycle through the control loop.

If the calculated mean value Pavg is no greater than the maximal meanvalue Pmax of the effective output power, the set point UPspt for the HFoutput voltage is incrementally increased as long as it remains smallerthan the prespecified value UPpsp. But if an increased UPspt wouldexceed UPpsp, the pulse duration PDact is increased instead, insofar asit is below the maximal pernissible pulse duration PDmax.

Regardless of whether the set point UPspt of the HF output voltage israised or lowered, or the pulse duration PDact is raised or lowered, thecycles through the control loop are repeated until the desired effect isachieved, namely the limitation of the effective output power of the HFgenerator to a maximal permissible value while the peak value of the HFoutput voltage is kept the same.

The modulation of the HF output voltage by means of a pulsed modulationsignal can be programmed in an alternative manner; that is, the onset ofmodulation or alteration of its pattern can be triggered when aparticular value of the load impedance has been reached, rather than themaximal mean value of the effective output power. For this purpose thevalue of the load impedance is continuously measured and evaluated.

The method in accordance with the invention for limiting the effectiveoutput power of a HF generator, as well as the HF generator designed forimplementing the method, are particularly suitable for use in HF surgeryinvolving the cutting and coagulation of human or animal tissue.However, it is also conceivable for the method and the generator to beemployed in any of the ways that HF generators can be used in otherareas of medicine or related fields.

At this juncture it should be pointed out that all the parts describedabove, in particular the details shown in the drawings, are claimed asessential to the invention in themselves individually as well as inevery combination. Modifications thereof are familiar to those skilledin the art.

1. High-frequency generator with adjustable limitation of the effectiveoutput power for HF surgical cutting of human or animal tissue,comprising: a device for finding the mean value of the effective outputpower of the HF generator, a comparison device for comparing the meanvalue thus found for the effective output power with a specified maximalmean value of the effective output power of the HF generator, amodulation device for modulating the output voltage of the HF generatorwith a modulation signal comprising pulses, such that a control devicefor controlling the modulation device alters the duration of the pulsesin the modulation signal and/or the duration of the intervals betweenthe pulses, in order to keep constant the peak value of the outputvoltage or the intensity of an arc that is formed between an electrodeconnected to the HF generator and the tissue, whenever the mean valuefound for the effective output power is greater than the maximal meanvalue of the effective output power.
 2. High-frequency generator asclaimed in claim 1, wherein at least two detector devices are disposedso as to detect the output voltage and output current of the HFgenerator, and that the device for finding the mean value of theeffective output power is constructed as an evaluation device and thepeak or effective values of the output voltage and the peak or effectivevalues of the output current are measured and used to calculate the meanvalue of the effective output power.
 3. High-frequency generator asclaimed in claim 1 wherein the control device comprises a limitingdevice to limit the range of alteration of the pulse duration so that itlies between a minimal permissible pulse duration and a maximalpermissible pulse duration-and/or to keep the duration of theinter-pulse interval between a minimal permissible interval duration anda maximal permissible interval duration.
 4. High-frequency generator asclaimed in claim 1, wherein the device measures the phase shift betweenthe output voltage and the output current.
 5. High-frequency generatoras claimed in claim 1, wherein an initialization device is soconstructed that it initializes prespecified peak values of the outputvoltage and the maximal permissible pulse duration and/or intervalduration to produce set points for the HF generator.
 6. High-frequencygenerator as claimed in claim 1, wherein the control device is incontrolling communication with a mains power supply that is soconstructed that the HF generator is supplied with higher power when themean value found for the effective output power is greater than thespecified maximal mean value of the effective output power. 7.High-frequency generator as claimed in claim 1, wherein the pulseduration and/or the interval duration is in a range from 3 μs to 200 ms.8. High-frequency generator as claimed in claim 1, wherein the controldevice is connected to a load impedance on the output side. 9.High-frequency generator as claimed in claim 1, wherein at least onearc-detector is present to detect the intensity of the electrical arcthat is formed between the electrode connected to the HF generator andthe tissue.
 10. High-frequency generator as claimed in claim 1, whereinthe integration time for finding the mean values of the effective outputpower or the effective values of the output voltage and/or the outputcurrent of the high-frequency generator corresponds to integralmultiples of the modulation cycle duration, i.e. the duration of onepulse plus the duration of one interval, but in any case corresponds toat least the duration of a single modulation cycle.
 11. Method forlimiting the effective output power of a high-frequency (HF) generatorfor HF surgical cutting and coagulation of human or animal tissue,comprising: finding the mean value of the effective output power of theHF generator by means of a specially designed device, comparing the meanvalue found for the effective output power with a specified maximal meanvalue of the effective output power of the HF generator by means of acomparison device, modulating the output voltage of the HF generatorwith a modulation signal comprising pulses, by means of a modulationdevice, and controlling the modulation device—by means of a controldevice in such a way that the duration of the pulses in the modulationsignal and/or the duration of the intervals between the pulses are/isaltered in order to keep constant the peak value of the output voltageand hence the intensity of an arc that is formed between an electrodeconnected to the HF generator and the tissue, whenever the mean valuefound for the effective output power is greater than the maximal meanvalue of the effective output power.
 12. Method according to claim 11,comprising the step of detecting the output voltage and the outputcurrent of the HF generator by means of at least two detector devicesand finding the peak or effective values of the output voltage and thepeak or effective values of the output current by means of the devicefor finding such values, here employed as an evaluation device. 13.Method as claimed in claim 11, comprising the step of limiting the rangeof alteration of the pulse duration so that it lies between a minimalpermissible pulse duration and a maximal permissible pulse durationand/or keeping the duration of the interval between pulses between aminimal permissible interval duration and a maximal permissible intervalduration.
 14. Method as claimed in claim 11, comprising the step ofmeasuring the phase shift between the output voltage and the outputcurrent in order to calculate the mean value of the effective outputpower.
 15. Method as claimed in claim 11 comprising the step ofinitializing a prespecified peak value of the output voltage and themaximal permissible pulse duration and/or interval duration to produceset points.
 16. Method as claimed in claim 15, comprising the step ofaltering the set point for the peak value of the output voltage when thepulse duration is not greater than the minimal permissible pulseduration.
 17. Method as claimed in claim 15, comprising the step ofaltering the set point for the peak value of the output voltage when theaverage value found for the effective output power is smaller than thespecified maximal mean value thereof, and when the set point is smallerthan the prespecified peak value.
 18. Method as claimed in claim 15,comprising the step of altering the pulse duration when the existingmean value is smaller than the specified maximal mean value of theeffective output power, and when the set point is not smaller than theprespecified peak value, and when the pulse duration is smaller than themaximal permissible pulse duration.
 19. Method as claimed in claim 11comprising the step of controlling a mains power device by means of thecontrol device in such a way that the mains power device—supplies the HFgenerator with higher power when the existing mean value is the same asor larger than the specified maximal mean value of the effective outputpower.
 20. Method as claimed in claim 11, wherein the pulse durationand/or the interval duration is in a range from 3 μs to 200 ms. 21.Method as claimed in claim 17, further comprising the step of detectingan intensity of an electrical arc that is formed between an electrodeconnected to the HF generator and the tissue, by means of at least onearc-detector device.
 22. Method as claimed in claim 11, wherein theintegration time for finding the mean values of the effective outputpower or the effective values of the output voltage and/or the outputcurrent of the high-frequency generator corresponds to integralmultiples of the modulation cycle duration.